1. Field of the Invention
The present invention relates to an organic light emitting material, more specifically, a copolymer having each monomer in its main chain, in which an iridium complex used as a phosphorescent material and a carbazole derivative having an excellent hole transporting capability are synthesized as the monomer.
2. Description of the Related Art
An organic light emitting device is a self-emissive display device that emits light by electrically exciting a fluorescent or phosphorescent organic compound. The organic light emitting device is advantageous in that it may be driven by a low voltage, manufactured in a thin film type, and has a wide viewing angle and fast response speed. Hence, the organic light emitting device can solve problems found in existing liquid crystal displays. Therefore, it has attracted attention as a next-generation display.
The organic light emitting device can be prepared by sequentially depositing a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, an electron injecting layer, and a cathode electrode on an anode electrode provided on a transparent substrate. All materials used in the organic light emitting device should have the preferred characteristics such that they have a high purity and are prepared by vacuum deposition. Further, the materials should exhibit high thermal stability at a glass transition temperature and thermal decomposition temperature, and be amorphous substances in order to prevent the deterioration of the device by crystallization due to Joule heat generated during device operation. The materials should also have good adhesion strength to other layers, but not move to other layers {Appl. Phys. Lett. 73, 1998, 729, Mat. science and Eng. R, 39, 2002, 143, Adva. Eng. Mat. Pro. Rep. 2002, 4, 453, Macromolecule 2003, 36, 9721, Adv. Mater. 2005, 17, 1109}.
Electrons in a compound may be in a singlet excitation state or a triplet excitation state. A probability of singlet state is a quarter, and the probability of triplet state is three quarters. Fluorescence occurs as the electron drops from the singlet state to the ground state, and phosphorescence occurs as the electron drops from the triplet state to the ground state. Therefore, the limit value of the internal quantum efficiency of light emitting material using fluorescence is 25%, and the limit value of the internal quantum efficiency of light emitting material using phosphorescence is 75%.
Further, the theoretical limit value of the internal quantum efficiency reaches to 100% in the energy transfer from the triplet excitation state to the singlet excitation state. The light emitting material, in which the luminous efficiency is improved by using such properties, is a phosphorescent material.
As the phosphorescent material, an organic metal compound is preferable, which has a transition metal with a high atomic number as a central atom undergoing intersystem crossing or energy transfer from the singlet or triplet excitation state to the triplet excitation state. It was found that as energy transfer is generated using an iridium compound as a sensitizer, the efficiency is rapidly increased {Inorg. chem. 1991. 30. 1685, NATURE 2000, 403 750-753, Appl. Phy. Lett. 77, 2000, 2280, Adv. Eng. Mat, 4, 2002, 453, J. Am. Chem. Soc. 2003, 125, 7379}.
In a single layer device having a structure of ITO(Indium-Tin-Oxide)/PVK(Poly-Vinylcarbazole)/Al, the maximum light emitting wavelength of purple color is 426 nm, and partially in the near UV region. A carbazole-based polymer is used to improve the luminous efficiency or color stabilization of organic light emitting device. In the case of using the system of blending other polymer with PVK (Polyvinylcabazole), the luminous efficiency is often rapidly increased, as compared to the case of using no PVK. In addition to a hole transporting characteristic, PVK forms a new light emitting species such as exciplex by interaction with a low molecular or high molecular weight material, thereby moving the light emitting wavelength {Syn. Mat. 1996. 80 271, Appl. Phys Lett 77, 2000, 2280, Adv. Mater. 2000, 12, 1949 Chem. Mater. 2004, 16, 442, Chem. Mater. 2004, 16, 4736}.
Many studies have been made on light emitting materials in the form of copolymer of the light emitting material and hole transporting material. It has been reported that the light emitting materials in the form of copolymer prevent phase separation and ionic aggregation, thereby capable of manufacturing a light emitting device, in which energy transfer to the light emitting material is efficient and stable, and its efficiency is excellent {Adv. Mater. 1999, 11, 285 J. Am. Chem. Soc. 2003, 125, 636, Organic Elctronics. 2003, 4, 105, Thin Solid Film. 2003, 353, 57, IEEE. 2004, 10, 115, Macromolecule 2006, 39, 349, J. Am. Chem. Soc. 2006, 128(20), 6647}.